1. Field of the Invention
The present invention relates to a bar code reader which optically reads information, and more particularly, an optical gain correction filter used for receiving reflected light holding information, and an optical apparatus having the same.
2. Description of the Related Art
Generally, information management techniques using symbol information such as a bar code are now widespread in all fields of industry, particularly in manufacturing and physical distribution. “Symbol information” means the combination of bars and spaces defined for recognizing optical information, or symbols or engraved bumps and dips or the like, for increasing the efficiency of inputting information.
As a bar code reader, a laser type reader using a semiconductor laser element, and a line sensor type reader using a line sensor element are known.
As a laser type bar code reader, a slot type scanner and a gun type scanner are known. In these bar code readers, a semiconductor laser light source is mounted, and a laser beam emitted from the light source is deflected and iteratively scanned by a drive mirror or the like. When the user radiates a scanning laser beam to the surface of a bar code stuck to an article, the light is scattered on the bar code surface, taken into a photodetector, converted to an electric signal, and decoded. The laser type bar code reader is useful in the circumstances where the distance from the reader to a bar code is relatively far, or the reader reads the bar code stuck to a moving article.
Contrarily, in the line sensor type bar code reader, a light source such as an LED is combined with an image pickup element such as a line sensor. As well known as a touch type scanner, when the user touches the image pickup area of a bar code reader to the illuminated surface of a bar code, the bar code image (an optical image) is formed by an image pickup lens on the light receiving surface of the line sensor, and taken in as an electric signal by opto-electric conversion. The electric signal is processed variously, and decoded.
Although these bar code readers are very similar in function, the reading method is different, as explained above. Due to the optical nature of a laser beam, the laser type bar code reader can read a bar code with relatively less degradation in the circumstances where the reader reads a bar code from a distant location.
The line sensor type bar code reader needs certain illumination, and degrades when reading a bar code from a distance. Thus, it is suitable for reading a relatively close bar code. However, the reading width (the angle of view) is wide, and it is necessary to correct a peripheral light intensity loss. Use of a line sensor enables so-called shading correction.
As a prior art of the above-mentioned types of bar code reader, a bar code reading range is broadened and optimized by making the shape of a laser beam spot cylindrical, as disclosed by Jpn. Pat. Appln. KOKAI Publication No. 8-55178. Further, as a prior art of the line sensor type bar code reader, a peripheral light intensity loss is corrected by adjusting the current supplied to each illumination LED, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-176185.
As described above, the aptitude of conventional bar code readers depends on the reading circumstances, for example, the distance to a bar code, and each reader is not constructed with general versatility. It is technically possible to mount both types of bar code reader in one system, but the size and weight increase, making it less easy to handle, and the cost rises. Because of these problems, a system having both types of bar code reader has not been produced.
Therefore, when the reading circumstances change after a system is built based on one reading method, the user cannot use a bar code reader for a different reading method. If the user wishes to use that bar code reader, the whole system must be modified, which is financially disadvantageous.
In addition, when a bar code reader is used in various fields and a plurality of bar code reading methods are introduced, the manufacturer must develop a plurality of similar modes, increasing an economical load in development, manufacturing and sales.
For example, when the line sensor type bar code reader is modified to be able to read a bar code from a distance, a peripheral light intensity loss arises, and an illumination light must be made brighter. And, when the number of illumination LEDS is increased for this purpose, the power consumption and product size increases, causing practical inconvenience.
On the other hand, the number of light sources may be only one in the bar code reader using a semiconductor laser light source, if it can be designed to be used also as a contact type reader. However, a peripheral light intensity loss, explained later, occurs in this type of bar code reader as a laser beam is scanned. The bar code reader of this type has only one semiconductor laser source, and it cannot correct a peripheral light intensity loss by the same method as that applied to the line sensor type bar code reader.
Now a peripheral light intensity loss will be explained.
Generally, in a light receiving element of an optical apparatus, a peripheral light intensity loss as conceptually indicated in FIG. 11 occurs. This is so-called cosine-to-the-fourth or lens vignetting, that is the phenomenon to decrease the receiving light quantity as the incident angle increases.
For example, assuming the angle of view necessary to read a bar code to be about 40°, as seen from FIG. 11, the signal obtained from an optical signal element is lost about 60% by a peripheral light intensity loss. This means that the bar code reading performance is extremely degraded.
Namely, when the peripheral light intensity loss is large, the light intensity fluctuates largely. In the analog-digital conversion in a binarization circuit by giving a threshold value, the above-mentioned signal with the peripheral light intensity loss is added to the signal corresponding to the pattern of a bar code, and these two signals coexist, degrading the binarization accuracy. For example, consider the case of setting a threshold value for binarization high in the signal processing circuit which binarizes the pattern of a bar code.
Even if the light intensity fluctuates largely due to a peripheral light intensity loss, a malfunction of the binarization circuit can be reduced, but for a bar code with a low contrast and a bar code with a high density, binarization becomes difficult and the bar code reading performance degrades. Contrarily, when the binarization threshold value is set small in the binarization circuit, the binarization is improved for the bar codes with the low contrast and high density. However, as for the periphery, where a peripheral light intensity loss occurs, the light intensity fluctuates largely and the binarization accuracy degrades. This results in extreme deterioration in the angle of view for reading. Similar degradation occurs under the reading conditions where the bar code signal amplitude becomes bad, for example, when the distance to a bar code is changed and defocused. Therefore, when the peripheral light intensity loss is large, the bar code reading performance is extremely degraded in general.